1. Field of the Invention
The present invention pertains to a process for the preparation of 1,1,1,2-tetrafluoroethane (HFC-134a). In particular it pertains to a process for the preparation of HFC-134a by a vapor phase catalyzed fluorination of 1,1,1-trifluoro-2-chloroethane (HCFC-133a) with hydrogen fluoride in a first reactor and flowing the resulting product to a second reactor together with trichloroethylene (TCE) and hydrogen fluoride (HF). The second reaction is conducted in the presence of a fluorination catalyst at a higher temperature than the first reaction.
2. Description of the Prior Art
It is known in the art that HFC-134a is a useful compound as a replacement for environmentally disadvantageous chlorofluorocarbon refrigerants. It is also useful as a blowing agent and as an aerosol propellant. Many methods for the production of HFC-134a are known in the art. U.S. Pat. Nos. 5,243,105 and 5,395,996 disclose a method of producing HFC-134a by a two step process which reacts trichloroethylene with hydrogen fluoride to form HCFC-133a. The HCFC-133a is then reacted with hydrogen fluoride in a second reaction to form HFC-134a. In these disclosures, the reaction of trichloroethylene with hydrogen fluoride to form HCFC-133a must be conducted at a lower temperature than the reaction of HCFC-133a with hydrogen fluoride. The reaction sequence and temperature differences are the reverse of those used in the present invention. U.S. Pat. Nos. 5,243,107 and 5,382,722 disclose the reaction of HCFC-133a and HF in a first reaction zone and then passes the reaction product to a second reaction zone together with trichlorethylene. This second reaction zone is at a lower temperature than the first reaction zone. Again, this is the opposite to the temperature difference sequence of the present invention. U.S. Pat. Nos. 5,334,786 and 5,395,998 produce HFC-134a by reacting trichloroethylene and hydrogen fluoride to produce HCFC-133a and then further fluorinate the HCFC-133a. The latter process requires a dilution of the trichloroethylene and hydrogen fluoride with nitrogen or argon gas which is inert to the reaction, and also requires three reactors for this process. U.S. Pat. No. 4,158,675 prepares 1,1,1,2-tetrafluoroethane HFC-134a by a vapor phase catalyzed fluorination of HCFC-133a with hydrogen fluoride in a first reactor. The reaction conditions produce unwanted 1,1-difluoro-2-chloroethylene as an impurity which is reacted with hydrogen fluoride.
It has been a problem in the art to achieve relatively high yields of HFC-134a without causing the simultaneous production of inordinate amounts of by-products which must be treated and disposed of safely. The present invention employs a method wherein intermediate mixtures are recycled through the production steps, thus increasing the efficiency of the process. The higher reactor temperature for the second reaction step affords several advantages. These include higher HCFC-133a productivity and higher TCE conversion. Therefore one can use a smaller reactor and less catalyst. Consequently, operating costs and capital investment are reduced. High conversions of trichloroethylene, approaching 100%, are made possible. Higher TCE conversion can eliminate the possibility of phase separation in the recycle stream. It also reduces amounts of TCE fed to the first reactor, which helps reduce the generation of HCl in the first reactor, and thus raises the equilibrium amounts of HFC-134a product formed in the first reactor. The amount of hydrochlorofluorocarbons as by-products is substantially reduced or eliminated. The process also produces useful HFC-125 and HFC-143a by-products instead of HCFC-123/124 and HCFC-141b/142b, respectively. The HFC-125 and HFC-143a, which are also useful refrigerants, have no ozone-depleting potential whereas HCFC-123/124 and 141b/142b have ozone-depleting potential and are being phased out. Energy is saved because refrigeration is not required to separate crude HFC-134a product from HCFC-133a and hydrogen fluoride that are recycled to the first reactor. In addition, since the reaction of trichloroethylene and hydrogen fluoride is exothermic, heat generated from this reaction is used to keep the second reactor at a higher temperature.